The use of macromolecular membranes as membrane materials for the separation and concentration of specified gases from gas mixtures has been widely studied and such membranes have been used in practical applications. Specifically, in the case of separation methods using such membrane materials, it is necessary that the membrane material used be superior in terms of gas permeability and selectivity for the gas that is to be separated and show stable characteristics in operation over a long period of time.
However, in the case of the conventional techniques above, it is not easy to separate water vapor from acid vapors such as vapors of acetic acid, formic acid, nitric acid or sulfuric acid which have a strong affinity for water vapor. Membrane materials which effectively block the passage of such acid vapors while allowing the passage of water vapor have not yet been obtained.